Piezoelectric materials have a widespread use in the medical field. The materials have for instance found use in electrodes and sensors for implantation in the human or animal body.
The piezoelectric materials used in the medical field for sensing purposes or for mechanical stimulation must meet high standards in regard of for instance sensitivity and durability. One consequence of this is that many prior art piezoelectric materials are less suitable for this purpose.
A piezoelectric material that is often used is lead zirconium titanate (PZT). However, this material has some recognized drawbacks and handling problems. Commonly occurring problems with PZT are inhomogeneous phases, reactions with the substrate, impurities of pyrochlore type and PbO formation at the surface. This is mainly due to the chemistry of lead: it has a low melting point and is easily reduced. This can lead to formation of Pb droplets in the material during the synthesis and shortage of Pb in the active material, which reduces the piezoelectricity. It can be noted, that an excess of Pb is often used in the synthesis to obtain the right composition in the PZT material. Further, PZT deposition on a Pt substrate (commonly used for implantation) is not recommended, since Pb alloys with Pt.
NKN (Sodium potassium niobate) does not exhibit these drawbacks. It does not alloy with Pt, and NKN can be heat treated at a higher temperature than PZT (NKN: 1000° C.; PZT: 500° C.).
WO99/54266 discloses a biocompatible ceramic material for implants comprising NaxKyNbO3, 0≦x≦0.8, 0.2≦y≦1, x+y=1. The object of this invention is to provide a long-term stable material that can be wholly or selectively polarized in order to obtain piezoelectric properties for tissue growth promoting purposes. WO99/53972 discloses a piezoelectric implant comprising NaxKyNbO3, 0≦x≦0.8, 0.2≦y≦1, x+y=1. The object of this disclosure is to provide an implant that has a sensitivity and a durability that meet the high standards required and which further is biocompatible.
Thus, NKN is known and has also shown excellent properties for use in implants. In addition, it is possible to polarize the material in order to provide it with piezoelectric properties. The material combines a very high level of biocompatibility, mechanical and chemical stability that are expected to be at least ten years, a piezoelectricity constant d33 that can exceed 100 pC/N, resistivity that can exceed 1012 Ωm, and a Curie temperature >160° C. The material will function as desired at a working temperature of 36-41° C., and a band width of 0.3-20 Hz. Thus, NKN is a highly desired piezoelectric material within this field
The conventional NKN-preparation methods include:
(1) calcinations and milling together with sintering, where however milling often brings contaminations from the milling equipment. Also, sintering may lead to oxygene defects in the material, which seem to be a result of the choice of sample holder during the sintering process. Moreover, sintering can e.g. be air-fired, hot pressed or made by hot isostatic pressure. The NKN-material can e.g. be made as a bulk material by means of the hot isostatic pressing methods using sodium carbonate, potassium carbonate and niobium pentoxide as precursors as defined in the following articles from American Ceramic Society Bulletin: Egerton-Dillon in 42 (1959) pp 438-442, Jaeger-Egerton in 45 (1962) pp 209-213 and Egerton-Bieling in 47 (1968) pp 1151-1156. Normally hot pressed materials give a higher d33-value (measure of piezoelectricity) than air-fired;(2) pulsed laser deposition (PLD) or laser ablation, having the drawback that Na- and K-compounds are volatile, which may lead to Na- and/or K-deficiency in the material (Self-assembling ferroelectric Na0.5K0.5NbO3 thin films by pulsed laser deposition” Choong-Rae Cho, Alex Grishin, Appl. Phys. Lett. 75, 268 (1999));(3) sputtering (e.g. Rf-magnetron sputtering), whereby the major drawback of Rf-magnetron is oxygene defects in the material, sometimes together with a Na2Nb4O11-contamination. The NKN-material may also be made in the form of films or layers on substrates by means of cathode sputtering methods as for instance described in Margolin et al, “(K, Na)NbO3 ferroelectric films synthesized by cathode sputtering”, Sov. Phys. Tech. Phys. 33(12), December 1988, or by other suitable thin film techniques;(4) solid-state reaction methods (sometimes with a subsequent hot pressing) (see e.g. Ichiki et al., Journal of the European Ceramic Society, 2004, 24; 6:1693-97). By using this method, the synthesis requires a relatively long time. Also, it is difficult to obtain a homogenous composition; or(5) chemical vapor deposition (CVD) (Choong-Rae, Materials Letters, 2002, 57; 4:781-786). In this disclosure, a NKN-film is deposited from precursors that are pre-evaporated at 700-750° C. However, the NKN material that is disclosed exhibit Nb deficiencies (the composition is estimated to comprise Na:K:Nb 1.00:1.00:1.47). Further, a mixture of NKN and the Si-substrate occurs in the interface, which results in a varying NKN composition.
With conventional NKN-production methods it is common with oxygene defects and other material problems. NKN having oxygene defects are often treated in oxygene in order to fill the defects, but that results in an additional production step, which makes the production more expensive. Also, for bulk material it is often difficult to remove the defects in the entire material.
Moreover, a common problem when using conventional synthesis methods for piezoelectric materials and NKN, is that the methods make it difficult to control the composition. This results in low phase purity, in a low piezoelectric effect, if any. Further, this makes it difficult to produce a thin piezoelectric film (which often is desirable in e.g. sensor applications) having reliable physical and chemical properties, since the material characteristics are varying and/or unpredictable.